In order to change the translation ratio of an oil-insulated transformer under load there are provided, depending on the respective purpose of use, so-called tap changers, on-load tap changers or off-load tap changers, whose configuration and mode of operation of which are known in principle and that are constructed for connecting the winding taps of a tap winding of a transformer.
An on-load tap changer of that kind connected with a tap winding usually comprises a load changeover switch and a selector, in which case the selector preferably consists of a preselector and a fine selector. The fine selector is provided for power-free selection of the winding tap that is to be connected, of the tap winding and the preselector for increasing the regulating range, wherein the preselector depending on the respective case of use can be constructed either as a reverser for the switching on and off of the tapped winding or as a coarse selector for coupling the tapped winding to the start or the end of the main winding of the transformer.
The load changeover itself is carried out by the load changeover switch of the on-load tap changer and, in particular, from the previous winding tap to the new, preselected winding tap of the tapped winding of the transformer. For that purpose, the load changeover switch usually comprises mechanical switching contacts and resistance contacts, wherein the switching contacts serve for direct connection of the respective winding taps with the load diverter and the resistance contacts serve for temporary bridging over by one or more switching-over resistances. However, developments in recent years have led away from load changeover switches with mechanical switching contacts and toward the use of vacuum interrupters or semiconductor switching elements, particularly power switching elements, as switching elements for high voltages and currents, for example thyristors, GTOs, IGBTs and other components.
Integrated on-load tap changers, also called load selectors, form a further category of on-load tap changers. Here the load changeover switch and the fine selector are incorporated in one unit. When actuated, in one step the winding tap to be connected is preselected and subsequently connected. This form of realization of an on-load tap changer can also comprise a preselector arranged either outside or within the housing of the on-load tap changer and, in particular, in an insulating medium, preferably insulating oil.
By contrast, off-load tap changers for tapped transformers serve for power-free, i.e. voltage-free or current-free, switching over between different winding taps of the tapped transformer connected with the off-load tap changer.
An off-load tap changer of that kind substantially consists of a plurality of contact rods circularly arranged around a center longitudinal axis and a central, rotatably mounted switching shaft to which a contact arrangement is attached. The contact arrangement serves for electrical contacting of two, preferably adjacent, contact rods with one another so as to thereby produce a defined transformation ratio of the transformer connected with the off-load tap changer.
In order to actuate both on-load tap changers and off-load tap changers these are usually connected by a linkage with a motor drive serving for setting the on- or off-load tap changer to the respectively desired operational setting. All essential mechanical and electrical subassemblies necessary for drive of the on- or off-load tap changer are combined in the motor drive. A motor drive of that kind also comprises, inter alia, a setting indicator with a switching step pointer and a setting pointer that are both actuated by a control drive and/or indicator drive. In that case, the setting indicator represents the currently activated setting of the on- or off-load tap changer, whereas the switching step pointer indicates whether an initiated switching-over process has in fact been completely executed. In such a case the switching step pointer will have run through a complete 360° rotation.
In that case, in the prior art the entire drive train of the on- or off-load tap changer up to the setting indicator in the motor drive is constructed as a continuous drive train that produces kinematic constrained movement of all individual subassemblies provided within this drive train. If the on- or off-load tap changer now has to be taken apart at specific time intervals for inspection purposes and serviced then after reinstallation it has to be ensured that the currently activated setting of the on- or off-load tap changer corresponds as before with the currently indicated setting of the setting indicator in the motor drive. In other words, the switch setting of the on- or off-load tap changer and that of the setting indicator have to coincide. For that purpose it is necessary with the prior art to release, i.e. take apart, the linkage that is fixedly connected with the motor drive in operation and that leads to the on- or off-load tap changer and to manually allow the motor drive to run until the switch setting indicated at the setting indicator corresponds with the actual setting activated by the on- or off-load tap changer. This manual readjustment is in that regard time-consuming and susceptible to error.